When a shaft is supported in a radial plain bearing, it always has an axial play, i.e. its axial position is not precisely defined. This axial play can be restricted by limiters. These limiters can take the form, for example, of steps on the shaft, or shaft rings. When these rest against an end face of a plain bearing bushing, they generate an additional frictional torque; and if such solutions are used in drive units with a varying utilization position or orientation, the axial play that is (still) present allows rattling. (The term “utilization position” refers to the position, e.g. horizontal or vertical, assumed by the motor during operation.)
In plain bearings with a vertical or almost vertical shaft position, so-called thrust bearings have proven successful. In these, the dead weight of the rotor presses via a dome-shaped shaft end onto a planar bearing disk and onto a small area thereon; and because of the small radius of the shaft end, only a relatively small frictional loss torque is generated and the axial shaft position is defined, but only if the drive unit is mounted in the intended utilization position or orientation.